Q4. What mass of oxalic acid dihydrate, H2C2O4·2H2O (126 g mol−1), makes 250 mL of a 0.0500 M standard solution?
0.0500 mol L−1 × 0.250 L = 0.0125 mol; 0.0125 × 126 = 1.5750 g, weighed accurately and made up to the mark in a 250 mL measuring flask.
Q5. What volume of 0.0200 M KMnO4 reacts with 10.0 mL of 0.0500 M Mohr’s salt?
M1V1 = 5M2V2: V2 = 0.0500 × 10.0 ÷ (5 × 0.0200) = 5.00 mL.
Q6. Assertion: in the oxalic acid titration the first few drops of permanganate lose their colour slowly, the later ones quickly. Reason: Mn2+ formed in the reaction catalyses it. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the manganous sulphate formed catalyses the reduction of permanganate by oxalic acid, so the rate rises as the titration goes on (Lab Manual page 41).
Q7. Why is about 5 mL of 1.0 M sulphuric acid added to the flask before titrating?
To keep the solution acidic, so that permanganate is reduced to Mn2+ and no manganese dioxide forms during the titration (Lab Manual page 42); in neutral or faintly alkaline solution it would stop at MnO2 (NCERT textbook page 108).